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0 pointstmoertel12y ago0 comments

(mapM_ print) produces a pure function that produces a single IO action, and (map print) produces a pure function that produces a list of IO actions. Neither has any effect unless called in a context that sequences the resultant actions into the main IO action that the runtime interprets (or a thread's action).

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mercurial12y ago· 2 in thread

I meant mapM_ in an IO action actually executed. On the other hand, getting a list of IO actions, even in the IO monad, doesn't amount to much.

tmoertelOP12y ago

And my point is that the action that mapM_ ultimately produces is not actually executed unless you sequence it into the main action (or a thread's action). Since mapM_'s eventual action is a singleton, you can do this sequencing with any combinator taking a singleton action, for example (>>) or (>>=), but it must be sequenced nonetheless, the same as for the list of actions that (map print) produces, if you want those actions to be executed.

mercurial12y ago

Fair enough.

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